This invention relates to a method of calibrating optical instruments and to a system of optical instruments which are calibrated in accordance with the method of the invention.
Several different kinds of optical instruments which analyze test samples by the reflectance or transmissivity of the samples at narrow bandwidths distributed throughout a spectrum are currently in use. These instruments are used to make quantitative measurements on agricultural samples, such as grain, meat, or hay and also are used to make measurements of the color of samples such as in the case of paint samples.
In one instrument of the prior art, light is dispersed into a spectrum by means of a grating which is oscillated at a relatively high rate of speed. Examples of this type of instrument are disclosed in U.S. Pat. Nos. of Landa 4,285,596 and 4,264,205. In another type of instrument, a plurality of interference filters are mounted on a filter wheel and each filter is rotated by the filter wheel sequentially into a path of collimated light. As the filter moves through a beam of collimated light, the angle of the filter to the incident light varies and the narrow bandwidth of light transmitted by the filter is changed. Examples of this latter type of instrument, which is called a tilting filter instrument, are disclosed in U.S. Pat. Nos. 3,861,788 to Donald R. Webster and 4,082,464 to Robert L. Johnson, Jr. In each of the above mentioned instruments, the narrow bandwidth of light transmitted by the instrument depends upon the angular position of an oscillating or rotating element. In the instrument with the oscillating grating, the transmitted bandwidth depends upon the angular position of the oscillating grating and outputs are obtained from the grating throughout a spectrum at two nanometer increments of the transmitted wavelength. In the tilting filter instrument, outputs are obtained at similarly small increments of the transmitted wavelengths.